An embodiment of a device for measuring body's core temperature is known from U.S. Pat. No. 6,220,750 B1. The known apparatus comprises a capsule, the inside of which is heat conductive, which has a surface adapted to be placed in contact with the surface of the body the core temperature of which is conceived to be measured, said capsule being thermally insulated on all its other surfaces. The known apparatus further comprises a heating element for heating the inside of the capsule and a temperature sensor for measuring the temperature inside the capsule, whereby the heating element is actuatable by a control unit arranged to switch on the heating element for compensating the thermal flux emanating from the body in an outwards direction.
The known apparatus is based on the insight that the body which temperature is to be measured has an inside core with a substantially constant temperature and an outer surface from which the heat is dissipated into the surrounding when the ambient temperature is lower than the body core temperature. It is further assumed that the body comprises an outer shell between the core and the surface wherein the temperature gradually decreases from that of the core to that of the surface. In the case of a steady heat flow and a constant conductivity throughout the shell, said temperature decreases. It is further assumed that since there are no heat sources in the body except the core, the temperature falls as a monotonous decreasing function from the core to the surface. Under these conditions, in steady state, if it is found that two intermediate points along a path between two extreme points have the same temperature, the two extreme points must be at the same temperature. The operation of the known device is based on the insight that if a path for heat flow can be created between the core of the body, the temperature of which is measured and points outside the body, and the flow of heat along this path can be controlled so that two points of said path are at the same temperature, under thermal equilibrium, this indicates that heat flow has ceased and their temperature will be the same as that of the core. This method is also referred to as a zero-flux method for determining the core temperature.
It is a disadvantage of the known apparatus that it is only usable for situation when the ambient temperature is lower than the body's core temperature.